This invention relates to a fuel system, and in particular to a fuel system suitable for use in aerospace applications.
FIG. 1 illustrates, diagrammatically, an aircraft engine fuel system which comprises an impellor pump 1 arranged to supply fuel from a relatively low pressure fuel source through a heat exchanger 2 and filter 3 to a high pressure gear pump 4. The high pressure gear pump 4 is typically arranged to increase the fuel pressure by over 200 psi. From the gear pump 4, fuel is supplied through a metering valve 5 and a pressure raising and shut off valve 6 to an engine. A spill valve is 7 arranged to return excess fuel to the inlet of the gear pump 4.
One or more variable stator vane actuators 8 are arranged to control the operation of various components of the aircraft engine. The actuators 8 are controlled by a servo-valve 9. The servo-valve 9 is arranged to receive fuel under pressure from the outlet of the gear pump 4, a return line from the servo-valve 9 being connected to the inlet side of the pump 4. In order to ensure that the servo-valve 9 and the actuators 8 operate correctly, it is important to ensure that the fuel pressure difference across the servo-valve 9 exceeds a predetermined pressure which is determined by, for example, the size of the actuators, and is typically of the order of 100 psi minimum. Where the gear pump 4 increases the fuel pressure by over 200 psi, then clearly the fuel pressure difference across the servo-valve 9 is sufficient to ensure that the servo-valve 9 and actuators 8 operate correctly.
It is desirable to reduce the number of heat exchangers used in aerospace systems, for example by omitting the oil/air heat exchangers used to cool the gearboxes and electrical generators associated with the engines, instead relying upon the oil/fuel heat exchangers to achieve such cooling. As the high pressure rise gear pumps used in the typical arrangements generate significant levels of heat, in order to allow the heat exchangers to remove as much heat as possible it is desirable to replace the high pressure gear rise pumps with units having a smaller pressure rise at low metered fuel flow rates to the engine, which generate less heat. As a result of the omission of one or more heat exchangers, a significant weight reduction can be made.
The use of a lower pump pressure rise may result in the fuel pressure difference across the servo-valve being insufficient to ensure that the servo-valve and actuators operate correctly, particularly when the engine, and hence the gear pump, is operating at low fuel flow rates and hence the gear pump spill (recirculation) flow rate is high. Although the dimensions of the servo-valve and the actuators can be increased to reduce the pressure necessary to achieve correct operation, such alterations to these components may result in them being unsuitable for use in aerospace applications due to their size and weight.
It is an object of the invention to provide a fuel system including a high pressure pump in which this disadvantage is obviated or mitigated.
According to the present invention there is provided a fuel system comprising a low pressure fuel pump having an outlet which is connected to the inlet of a high pressure pump arranged, in use, to supply fuel to an engine, and at least one actuator controlled by a servo-valve, the servo-valve having a high pressure connection connected to the high pressure side of the high pressure pump and a low pressure connection connected upstream of the outlet of the low pressure pump.
As the low pressure connection of the servo-valve is connected upstream of the outlet of the low pressure pump rather than to the low pressure side of the high pressure pump, a pressure difference can be achieved across the servo-valve which is greater than the pressure rise generated by the high pressure pump. It will be appreciated, therefore, that a high pressure pump arranged to produce a lower minimum pressure rise may be used without impairing the performance of the servo-valve and associated actuators.
The high pressure pump conveniently comprises a gear pump.
In order to reduce the risk of hot fuel within the low pressure connection vapourising, which may result in the low pressure pump becoming vapour-locked if it takes the form of an impellor pump, a cooling flow of fuel may be supplied to the low pressure connection from the low pressure side of the high pressure pump before any series heat exchanger. In order to avoid vaporisation in the part of the low pressure connection upstream of the part receiving the cooling flow, a valve may be provided in the low pressure connection, the valve maintaining the fuel pressure at a level sufficient to ensure that vaporisation does not occur. Alternatively, the low pressure connection may be connected to a part of the low pressure pump in which the fuel pressure is at a level intermediate the inlet and outlet pressures of the low pressure pump and sufficient to avoid vaporisation of fuel.